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P0300 Misfires..Brain Teaser

Old 07-04-2019, 11:16 AM
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P0300 Misfires..Brain Teaser

2007 Boxster S,

Hey guys, this thing is draining my brain.

Let me run past what I have done so far.
Car originally was leaking engine oil due to a broken chain guide bolt on bank 1 (O/S) 1-3. No running faults despite the chain on that bank being pretty loose.

I removed the engine for better access, removed the broken bolt, chain guides were broken so fitted new chain guides, tensioner and re-assembled.

No disassembly of bank 2 at all nor any inlet plenum of bank 1. Rotated over only clockwise

Started it up and road test found no problems.

Ran ok around 40 miles when EML came on and misfire.

Code read cyl P0300 and 1+2+3 misfire.

Of course common for valve lift solenoid, so swapped valve from N/S and fault remained on O/S.

Started to re-check the work. If being really critical, adjusted exhaust valve timing 2 degrees on bank that had been touched

Re-assembled and test drive. After a few miles EML back on and misfire. But now on 4, 5,6. No fault on bank 1 at all.

Fault only happens when warm and only sub 2500rpm. If driven continuously above 2500 fault not apparent. No EML
Actuation of cam timing and valve lift solenoids has expected results. Both cam and lift solenoids have been swapped banks. Cam sensor signals are within spec of each other. Switched cam sensors over as well.
Coils have been changed bank to bank
Plugs ok
Smoke test passed ok
Vac system and diaphrams all working ok
Scope test all passed, crank + cam sensors, knock sensors,
MAF as expected in times of misfire and non misfire state
O2 sensors ok
CO/HC and lambda readings are pretty much identical on each bank whilst running in no fault condition (sub 60C)
Compression ok
Fuel pressure ok

Misfire counters are being triggered on 4,5,6. Zero on 1,2,3

Car has no audible misfire. Only after a few minutes and whilst 60 degrees plus, lambda on bank 2 goes lean and stays so. Slight misfire can then be heard to start. Few moments later serious misfire occurs. Counters of all 3 have risen significantly. Then one cylinder goes very high whilst other 2 are reset to zero.

When EML is flashing but it is not yet in the serious misfire state, then raising the engine speed to over 2500 stops the increase in misfire counter until they return to zero and EML goes out. Once revs are dropped to under 2500 then counters re-start.

The chosen cylinder that stays high seems to be randomly chosen by ecu. 4 or 5 or 6

As many of you know, misfire counters are triggered by crank sensor. Disconnect crank sensor and start (using cam sensors as ref) and no misfire or fault occurs, although this means no variocam etc works.
Crank sensor scoped ok and I did in fact change it from another working ok boxster. Also have switched cam sensors bank to bank. Flywheel missing tooth in correct position.

Am I missing something?
Old 07-12-2019, 02:51 PM
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Macster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond reputeMacster has a reputation beyond repute
If the toothed ring the flywheel has which the crankshaft position sensor relies upon to know the position of the pistons has a "tooth" missing that's not good. An intact and proper ring needs to be fitted.

Can't believe the flywheel has a gear tooth missing. But if it does I'd replace the flywheel. A missing tooth represents an imbalance that could certainly account for some untoward behavior maybe even the behavior you are seeing.
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